1. Field of the Invention
The present invention relates to a method for manufacturing a conductive structure. In more detail, the present invention relates to a method for manufacturing a electroconductive structure made of crystalline thermoplastic resin composite material containing conductive filler material and having high electrical conductivity and heat resistance obtained by increasing the degree of crystallinity of the composite material, and to a method for manufacturing a separator for a fuel cell.
2. Description of Related Art
There have conventionally been used mainly metallic materials and/or carbon materials, etc., for applications that require high electrical conductivity. However, conductive resin compositions are likely to play a much larger role as one kind of conductive material due to the recent diversified applications of conductive materials in various fields such as electronics, electrochemistry, energy, and transport machinery. Accordingly, conductive resin compositions have achieved a remarkable breakthrough in terms of performance and functionality. As an important factor in this breakthrough a substantial improvement in molding processability due to composition of polymer materials can be mentioned.
It is important for conductive resin compositions to effectively develop electrical conductivity without practically losing mechanical characteristics and molding processability, etc. As applications that require electrical conductivity other than conventional ones are recently cited, for example, electronic materials such as circuit boards, resistors, laminates, and electrodes, heaters, pyrogen unit members, filter elements for dust collection, PTC elements, electronics parts, and semiconductor parts can be mentioned. In these applications, not only electrical conductivity but also high heat resistance is required.
Meanwhile, fuel cells have received widespread attention recently from the aspects of environmental issues and energy issues, etc. Fuel cells are clean generators that utilize hydrogen and oxygen to generate electricity through a reverse electrolytic process with no emissions other than water. Also in the field of fuel cells, conductive resin compositions can play an important role. Among several kinds of fuel cells according to type of electrolytes, proton-exchange membrane fuel cells are most promising for automotive and consumer use due to their low-temperature operability. Such fuel cells can achieve high-power generation by stacking unit cells composed of, for example, a polymer solid electrolyte, gas diffusion electrode, catalyst, and separator.
In a separator for separation within a single cell of thus arranged fuel cells is generally formed a flow path (groove) for supplying fuel gas (e.g. hydrogen) and oxidizing gas (e.g. oxygen) and for discharging generated moisture (water vapor) therethrough. Such a separator is therefore required to have high gas impermeability for complete separation of these gases and high electrical conductivity for reduced internal resistance. Such a separator is further required to have high heat conductivity, durability and strength.
In order to achieve the requirements above, there has conventionally been considered the possibility of using metallic materials or carbon materials for such a separator for a fuel cell. In respect to metallic materials of these materials, although it has been attempted to coat the surface thereof with precious metal or carbon in consideration of their poor corrosion resistances, there is a problem of insufficient durability and increased cost in coating.
Meanwhile, there have also been significant consideration given to carbon materials, and as a separator material for a fuel cell that can be mentioned, for example, a molded product obtained by press molding an exfoliated graphite sheet, a molded product hardened by impregnating a carbon sintered body with resin, and glassy carbon obtained by baking thermosetting resin, a molded product obtained by molding the mixture of carbon powder and resin can be mentioned.
For example, Patent Document 1 discloses a complex process comprising the steps of: adding bonding material to carbonaceous powder to perform heat mixing treatment and then CIP (Cold Isostatic Pressing) molding; baking and graphitizing the powder; impregnating and hardening the obtained isotropic graphite material with thermosetting resin; and cutting a gas flow groove by a cutting operation.
It has also been attempted to improve the performance of separators by changing the composition thereof. For example, Patent Document 2 discloses a separator having superior mechanical and electrical characteristics obtained by a composition of carbonaceous powder coated with resin and resin with higher heat resistance than that of the coating resin.
Patent Document 3 discloses a resin composition made of a mixture of low-melting metal, metal powder, thermoplastic resin, and thermoplastic elastomer.
Meanwhile, it has also been attempted to manufacture high-performance separators using a simple method by changing (arranging) the molding method thereof. For example, Patent Document 4 discloses a method for manufacturing a conductive molded product comprising the steps of: preliminarily heating a mold to the melting point of thermoplastic resin or higher; filling the mold cavity with a heated conductive composition; melting and shaping the composition in a compressive manner evenly at a predetermined pressure; and cooling the composition while applying pressure to the mold to below the thermal deformation temperature of the thermoplastic resin. Also Patent Documents 5 and 6 disclose methods of molding a highly conductive resin molded product characterized by setting the cavity surface temperature higher than a temperature that is 50° C. lower than the crystallization temperature of a thermoplastic resin composition and lower than the melting point of the composition to perform injection molding.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. Hei 8-222241
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2003-257446
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2000-348739
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2003-109622
Patent Document 5: Japanese Unexamined Patent Application, First Publication No. 2004-35826
Patent Document 6: Japanese Unexamined Patent Application, First Publication No. 2004-34611